Conventionally, there is provided an LED headlamp lighting device for vehicle (see, e.g., Japanese Patent Application Publication No. 2011-113642). The LED headlamp lighting device includes a DC/DC converter which steps up/down a DC voltage supplied from a battery in response to turning on of a LOW beam switch into a DC voltage of a desired voltage value, and a control circuit which controls the on/off of switching elements of the DC/DC converter. The LED headlamp lighting device further includes a power detection circuit to detect a source voltage of the battery and a current detection circuit to detect a current flowing through light emitting diodes (LEDs). The control circuit includes a lamp current command value operation unit and a comparison operation unit.
The lamp current command value operation unit selects a lamp current command value stored in a storage unit in advance based on the source voltage, and outputs it as the lamp current command value. The comparison operation unit compares the lamp current command value outputted from the lamp current command value operation unit with an output current value detected by the current detection circuit, and calculates and outputs an output current command value to make these values equal. Thus, in the LED headlamp lighting device, constant current control can be achieved by controlling the output current command value outputted from the comparison operation unit.
In the LED headlamp lighting device, if the source voltage of the battery is reduced to, e.g., less than 8 V, it begins to decrease the output current. Then, the reduction of output current is performed depending on a change in the source voltage, and control is performed to maintain a rated current if the source voltage is equal to or greater than 8 V. By this control, it is possible to reduce an increase in circuit loss at a low source voltage, and prevent damage to the lighting device.
In the LED headlamp lighting device disclosed in Japanese Patent Application Publication No. 2011-113642, a plurality of LEDs connected in series are used as a load. In this case, there is a variation in luminous flux between the LEDs, and its effect appears prominently when the same forward current flows in each LED. On the other hand, the output current command value is uniquely determined by, e.g., a data table prepared in advance. Accordingly, even if the constant current control is performed using the same output current in multiple lighting devices, the LED brightness is different between the lighting devices or LEDs.
Therefore, in order to set the brightness of each LED in the multiple lighting devices to a predetermined range, it is necessary to select an LED having a luminous flux within a predetermined range. In this case, since the selection process of LEDs is needed to be carried out by a manufacturer of the LEDs, the cost of LEDs increases. The LED brightness cannot be finely controlled to the extent that a user desires.
In order to solve the above-mentioned problems, multiple data tables can be prepared in advance, and a data table can be selected according to the luminous flux of the LED. This eliminates a need for the selection of the LED, thereby suppressing an increase in cost of LEDs. However, in this case, since it requires a memory with a capacity to store the multiple data tables, it may cause an increase in cost.